Despite grounding all the planes for nearly five months in 2011 and continued investigation since, Air Force engineers found a few "contributing factors" but haven't found anything to fully explain why, on nearly two dozen occasions, pilots have reported "hypoxia-like" symptoms in mid-air, Lt. Gen. Herbert Carlisle said Tuesday, according to multiple reports. Carlisle is one of the highest-ranking Pentagon officials to publicly discuss details of the F-22s' apparent oxygen problems.

"We have looked at everything on that system [to] the nth degree, and the bottom line is that there's no smoking gun," said Carlisle, the Air Force's chief of staff for operations, plans and requirements, according to the Air Force Times.

The three-star lieutenant general did say the engineers found issues with leaks in the jets' cooling system and problems with the breathing regulator valve on the pilots' mask but could not say they were the main culprits, the military blog DoD Buzz reported. Another Air Force official involved in the F-22 program told ABC News today he agreed with Carlisle's assessment and stressed the incidents are exceedingly rare - less than two dozen incidents out of thousands of otherwise successful training missions.

Just hours after Carlisle's comments, ABC News reported Tuesday a military base in Alaska, Joint Base Elmendorf-Richardson, had previously issued a single-day grounding for their F-22s after pilots reported three incidents of experiencing "hypoxia-like" symptoms in just the last two weeks. In two of those cases the pilots had to activate their back-up emergency oxygen system but in each case the pilots returned safely to base, an Air Force official at the base told ABC News. Hypoxia occurs when the brain does not get enough oxygen and can cause dizziness, confusion, poor judgment and inattentiveness.

The planes went back in the air the next day after a "review" of the incidents.

The F-22 Raptor, America's most expensive and sophisticated fighter jet - the fleet of which cost the U.S. government an estimated $77.4 billion - has yet to be flown in combat operations even though the planes went combat ready in late 2005. From Iraq and Afghanistan to Libya, the Air Force said the planes simply have not been an operational requirement.